1. Field of the Invention
The present invention relates to active matrix display apparatuses having a flat panel structure, typified by liquid crystal displays (LCDs), and driving methods for the display apparatuses. More particularly, it relates to a configuration of a counter electrode that faces pixel electrodes integratedly formed in a matrix form and a driving method for the counter electrode.
2. Description of the Related Art
FIG. 6 is a circuit block diagram schematically showing an example of a known display apparatus. Referring to FIG. 6, the display apparatus basically includes a pixel array unit 1, vertical direction shift registers 2a, and a horizontal direction shift register 3a. The pixel array unit 1 includes scanning lines X arranged in rows, signal lines Y arranged in columns, and pixels 5 arranged in a matrix form in association with intersections of the scanning lines X and the signal lines Y. The vertical direction shift registers 2a are arranged at the left and right of the pixel array unit 1 to drive the pixel array unit 1 from the left and right at the same time. More specifically, the vertical direction shift registers 2a sequentially apply selection pulses to the scanning lines X so as to sequentially select pixels 5 row by row. The horizontal direction shift register 3a applies a video signal VIDEO whose polarity inverts between positive and negative with respect to a predetermined reference potential COM to each of the signal lines Y, and writes the signal VIDEO whose polarity is positive or negative in the pixels 5 in a selected row. More specifically, the horizontal direction shift register 3a sequentially opens and closes a horizontal switch HSW connected to an upper end of each of the signal lines Y. The horizontal switch HSW connects each of the signal lines Y to a common video line 3b. The video signal VIDEO is externally supplied to the video line 3b. The horizontal direction shift register 3a sequentially opens and closes the horizontal switch HSW to sample the signal VIDEO at each of the signal lines Y.
Each of the pixels 5 includes a switching element composed of a transistor Tr; and a pixel electrode 5a. The transistor Tr is connected to the corresponding scanning line X and signal line Y and is switched on in accordance with a selection pulse. A signal VIDEO is written in the pixel electrode 5a via the switched-on transistor Tr. The signal VIDEO is sampled at the signal line Y by the horizontal direction shift register 3a. Furthermore, a counter electrode 21 is arranged facing the pixel electrode 5a with a predetermined space therebetween. The counter electrode 21 is common for all the pixel electrodes 5a. For example, liquid crystal functioning as an electro-optic material is held between the counter electrode 21 and the pixel electrode 5a, and a liquid crystal cell LC is formed for each pixel. The optical characteristics of the liquid crystal cell LC changes based on a potential difference between the pixel electrode 5a and the counter electrode 21, so that desired image display is performed. Each of the pixels 5 further includes an auxiliary capacitor Cs for holding a signal written in the pixel electrode 5a. One electrode of the auxiliary capacitor Cs is connected to a corresponding transistor Tr, and the other electrode of the auxiliary capacitors Cs is fixed to a reference potential COM via an auxiliary capacitor line Xcs. The counter electrode 21 is also fixed to the same reference potential COM.
FIG. 7 is a schematic diagram showing a driving method for the display apparatus shown in FIG. 6. A so-called 1 H inversion driving method and a so-called 1 F inversion driving method are adopted. The active matrix display apparatus has a flat panel structure and includes a pixel substrate 10 and a counter substrate 20 joined together with a predetermined space therebetween. For example, liquid crystal functioning as an electro-optic material is held in the space between the pixel substrate 10 and the counter substrate 20. The pixel electrodes 5a are arranged in a matrix form on the pixel substrate 10. For simpler explanation, the pixel array unit 1 is shown by 4×5 pixels. In contrast, the counter electrode 21 is arranged as one solid unit over the counter substrate 20. The counter electrode 21 is fixed at the predetermined reference potential COM, for example, COM=7.5 V.
In the first field, at the first horizontal period, a high (H) signal with respect to the reference potential COM is written in pixels in the first row. This signal level is, for example, 12.5 to 7.5 V. At the next horizontal period, a signal whose polarity is inverted to low (L) is written in pixels in the second row. The level of the low signal is 2.5 to 7.5 V. Since the polarity of a signal written in a pixel row inverts horizontal period by horizontal period (1H), as described above, this method is called 1 H inversion driving. Similarly, 1 H inversion driving is performed in the second field. However, when attention is focused on a pixel row, the polarity of the signal written in the first field is different from the polarity of the signal written in the second field. For example, when attention is focused on pixels in the first row, a signal at H level is written in the first field, and in contrast, a signal at L level is written in the second field. Accordingly, since the polarity of a signal written in pixels is inverted field by field (1 F), this method is called 1 F inversion driving.
Such active matrix display apparatus driving methods are disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2002-107693 and Japanese Unexamined Patent Application Publication No. 2003-5151.
As shown in FIG. 7, in the known display apparatus, the counter substrate 20 has a common potential and a solid structure. On the pixel substrate 10, in the first field, a signal potential inverts row by row, such as H, L, H, and L, and in the second field, the phase is inverted and a signal potential inverts row by row, such as L, H, L, and H, so that trouble in image quality, such as flickering, can be prevented. However, in 1 H inversion driving, the polarity of a signal potential in the first row is opposite to the polarity of a signal potential in the second row. Thus, for example, when the signal amplitude is 5.0 V, a potential difference of at most 10.0 V generates in a space (a) between the pixels. In contrast, a voltage of at most 5.0 V is applied between the pixel substrate 10 and the counter substrate 20. For example, assuming that the space between the pixel substrate 10 and the counter substrate 20 is approximately 3 μm, even if the size of the space (a) between the pixels is 3 μm, the electric field intensity between the pixels is approximately twice the counter substrate 20. Thus, orientation of liquid crystal at an end of a pixel electrode is disordered. In order to conceal the orientation disorder, a light-shielded area, such as a black mask, must be enlarged. However, this causes a reduction in the pixel opening ratio. This tendency has a larger influence due to an increase in the density of pixels. Thus, in the current situation, a phenomenon (hysteresis) in which liquid crystal molecules are displaced too far to return to the original due to a lateral electric field between pixels occurs. As described above, an increase in the density of pixels causes a problem, such as orientation disorder due to a lateral electric field between the pixels. This is because a lateral electric field between adjacent pixels is stronger than a vertical electric field between a pixel substrate and a counter electrode. As a result of this, problems, such as a reduction in the contrast due to orientation disorder, a reduction in the transmittance due to an increase in a light-shielded area to conceal the orientation disorder, hysteresis of liquid crystal molecules due to local concentration of an electric field, and the like occur. In accordance with an increase in the density, reducing the intensity in an electric field between adjacent pixels is becoming a more important issue.
In accordance with an increase in the signal amplitude, the intensity of an electric field operating between pixels is increased, and orientation of liquid crystal is disordered. In addition, large signal amplitude causes various problems. For example, noise caused by a signal change largely affects pixel potential via parasitic capacitance and this causes inferior image quality, such as crosstalk and blurring or ghost images when a window is displayed. Also, large signal amplitude causes a large difference between a pixel potential and a signal line potential, and significant leakage of a transistor occurs. For example, a problem, such as a reduction in the image quality due to light leakage, is caused.
In order to reduce signal amplitude by half, a VCOM inversion driving method has been proposed. This is a method for inverting a voltage VCOM applied to a counter electrode at a 1 H period and for inverting, in accordance with this, a signal potential written in a pixel electrode. In principle, VCOM inversion driving is capable of reducing signal amplitude by half compared with a case where the potential of a counter electrode is fixed. However, actually, inversion driving of a counter electrode formed as one solid unit having a large capacity at a high speed period of 1 H is difficult, and this is not practical as a solving means.